Analog to digital converters (ADCs) convert analog signals to digital signals. In some classes of ADCs, such as ramp converters, a voltage ramp is compared with the analog input using a comparator. When the voltage of the voltage ramp is equal to the analog input signal, the comparator generates a zero crossing signal and causes the digital signal to be generated. In these architectures, it is important to generate accurate ramp voltages because the ramp voltage is compared to the analog signal to generate the digital signal.
Many of the circuits that generate ramp voltages rely on a constant current source that charges capacitors. For example, a constant current source charges a capacitor wherein the voltage across the charging capacitor is the ramp voltage. The slope of the ramp voltage depends upon the absolute value of the current source and value of the capacitor. In order to generate highly accurate ramp voltages, the current source and/or the capacitor needs to be trimmed very accurately, which is difficult and time consuming.